Current and frequency control system for electrical discharge machining apparatus

ABSTRACT

An electrical discharge machining circuit of independently variable machining pulse frequency and current magnitude as set by machining pulse on-time. A current control system is employed to limit current to the gap to a safe level in accordance with each preselected frequency and current magnitude combination. Included to accomplish the control function are a plurality of gating means connected in a logic system and employing the resistor-transistor configuration.

United States Patent 1 Bell, Jr.

1 CURRENT AND FREQUENCY CoNTRoL SYSTEM FOR ELECTRICAL DISCHARGEMACHINING APPARATUS inventor: Oliver A. .Bell, Jr Mooresville, A N.C.

Assigneez Elox lnc., Davidson, N.C.

Filed: 1 Sept. 13, 1 971 Appl. No}: 179,6 64

us. Cl. ..219/69 C ..B23p 1/08 Field 61s611rh...;.., ..219/69 C, 69 G,69 P, 219/69 R, 69 s, 69 v [56] References Cited UNITED STATES PATENTS3,590,317 6/1971 Sennowitz ..219/69 C X i HLE, A k 1,

[ 51 June 5, 1973 3,213,258 10/1965 ZFerguson 2l9,/69P

Primary Examiner-R. F, Staubly V i Attorney-Robert C. I-lauk'e, Ernestl. Gifford, Claude A. Patalidis et al., I

57 ABSTRACT T1 14 Claims, 5 Drawing Figures PATENTED 5 \975 3, 737. 6 l5- SHEET 2 OF 4 IN V EN TOR.

OL/VER 4951.1. JR. F/G -2 y PATENIEDJUH 51913 3,737,615

SHEET 3 BF 4 INVENTOR. F7613 OLIVER A. BELL JR BY $4 aymgm CURRENTANDFREQUENCY CONTROL SYSTEM FOR ELECTRICAL DISCHARGE MACHINING APPARATUSBACKGROUND OF THE INVENTION Circuits for electrical discharge machiningapparatus typically incorporate a number of electronic output switcheswhich are operated singly or parallel connected in banks to providemachining power pulses across a dielectric filled machining gap.Material is removed from an electrically conductive workpiece coincidentwith the gap dielectric breakdown each time a machining power pulsepasses across the gap. In order to insure'repeatability' andcontrollability of machining operation, the power supply circuitgenerally incorporates a pulsing means or pulse generator whichoperatesthe electronic output switches, turning them on and off atprecisely controllable frequencies and at controllable. currentmagnitude as maintained by pulse on-time duration. The use of solidstate devices, such as transistors, as the electronic output switchesgives rise to a problem since these devices are being switched backandforth between a non-conductive state and a conductive state atrelatively highfrequencies, particuilarly'in finishing operations. Thepower dissipated dur- ;.in'g thisswitching,'added to the power generatedwhile the? transistor is in its conductive state, may cause excessiveheating and increase the possibility of a secondary breakdown occurring.To prevent possible damage v Lto the'transistor from this heating effectand to prevent damage to the gap elements, it is necessary in some way:to limit the current passed through the transistor yet still permittingoperation at the high frequency desired. Various prior art systems havefailed to directly solve thi sproblem. 3T hose systems whereby power isinterrupted after the fact" and after the transistor has actuallyfailedare not satisfactory.

' BRIEF DESCRIPTION OF THE INVENTION 7 Thepresent invention incorporatesa current control system which takes its inputs from simple deck typeing frequency and pulse current magnitude, i.e. ontime, are adjusted.This current limiting is accomplished through the operation of a logiccircuit which includes a plurality of NOR type gating means. It isimportant that while the control system limits the maximumcu'rrent wherethis is necessary it also permits norm'alzoperation for lowerfrequencies. The system ac- :cording to thepresent invention takesinputs from its i 'own pulser control and then connects the appropriate-magnitude resistor combinations in the gap circuit. 4:. The controlsystem is adaptable to receive inputs 'fromanoth'er machining powersupply such as when the electrical discharge machining apparatus isbeing operated in a dual system, such as where two power supplies "areused to provide machining power pulses to a single machining gap. Itwill be seen that the present invention is not limited to EDM circuitsemploying transistors or other solid state switching devices, but isapplicable to any EDM circuit using electronic switchesto deliverpower'pulses to the gap. The term electronic switch is used to mean anyelectronic control device having three or more electrodes comprising atleast two principal or power conducting electrodes acting to controlcurrent flow in the power circuit, the conductivity of the power circuitbeing controlled by a control electrode within the switch whereby theconductivity of the I power circuit is controlled statically orelectrically without the movement of any mechanical elements within theswitch. Included within this definition by way of illustration butynotlimitation are vacuum tubes,

transistors, semi-conductor controlled switches, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention, its operationalfeatures and its advantages will be understood from the followingspecification taken together with the drawings inwhich like numerals areused to refer to identical parts throughout the several views, andwherein:

FIG. 1 is a combined schematic and block' diagrammatic showing of anelectrical discharge machining power supply incorporating the presentinvention;

FIG. 1a is aschematic drawingshowing the variable impedance network 24of FIG. 1; and FIGS. 2, 3 and4 are schematic drawings of'the cur.- rentcontrol system used..

DETAILED DESCRIPTIONOF THE PREFERRED EMBODIMENT Also included in theelectrical discharge machining power supply is a pulsing means fortheelectronic output switches 12, 14 indicated generally by the numeral20. In this case, the pulse generator shown is an astable multivibratorof the type well known'in the electrical discharge machining art.Connected intermediate the multivibrator 20 and the control electrode ofthe output transistors 12, 14 are one or more drive stages 22, whichstages are used to shape and amplify the triggering pulses beingprovided to operate the output transistors l2, 14. The gap circuit willalso be seen to include a variable impedance means 24 connected betweenthe positive terminal of the machining power source 1 0 and theelectrode 16. A plurality of resistors 26,28, 30, 32, 34, 36 and 38 areconnectable in series and series parallel relationship with the gapaccording to the selective energization of one or more of a set ofcontrol relays.

provided representative of the frequency settings of the multivibrator20 through a switch 54 and a corresponding deck switch 54a and with theother inputs received representative of the pulse on-time, i.e. currentmagnitude, preset for multivibrator 20 through a corresponding deckswitch 56a. The power supply illustrated in the drawing of FIG. 1includes provision for ID different frequency or tap settings and atleast a like number of current magnitude settings.

Attention now will be given to the mode of operation of themultivibrator 20 and the manner in which the inputs of the currentcontrol system 40 were taken in accordance with the frequency andon-time settings used to control multivibrator operation. Themultivibrator 20 includes as its major componenta pair of alternatelyoperable transistors 42 and 46, each of which has coupled to it aDarlington connected transistor 48, 50 to increase the magnitude of thepower output. In accor dance with normal astable multivibrator design,the frequency of operation is controlled by the relative magnitude ofthe resistor-capacitor combinations. In the return networks, asindicatedin FIG. 1, there are ten frequency settings indicated by theletters S S Frequency is selected by the operation of the gangedswitches 52 and 54. The current magnitude of the machining power pulsesis controlled by the setting of the two potentiometers 56 and 58 whichtypically may be ganged for operation. In the embodiment shown, thelower frequency settings have their on-off time controlled by thepotentiometer 56, whilethe higher frequency settings at the lowerportion of thediagram are controlled by the setting of the potentiometer58.

The output for the drive stage 22 and the subsequent electronic outputswitch stage, including the transistors 12, 14, is taken from thecollector of the transistor 46. A reduced number of frequency inputsfive in number are taken from the wafer deck switch 54a which switch isoperated in conjunction with switches 52, 54 in such mannerthatfrequency inputs F F are available to the current control system 40at the same time that a plurality of current magnitude inputs areprovided through the terminals identified as C C in accordance with thecurrent magnitude or on-time settings which have been made controllingthemultivibra- .tor operation through potentiometers 56 and 58. The

multivibrator 20 further has its own DC voltage source 59, with seriesresistors 59, 61 connected in the manner shown and with diodes 62, 64poled to permit current flow in the directions indicated.

The manner in which the system operates will now be explained withparticular reference to the schematic drawing of FIG. 2. The FIG. 2drawing shows the inputs taken at the left hand side of the drawing.Each input is received through a filter network comprising a capacitor70 and a resistor 72. With respect to the various gating devices used inthedrawings of FIGS. 2, 3 and 4, these are preferably integrated circuitpositive logic gates of the NOR type currently used in the art. Withrespect to certain of the components illustrated, for example, thetwo-input gates numbered 101, 103 through 110, 112, 116 through 119,these are of the type manufactured and sold by Motorola, Inc. anddesignated by their Model No. MC724P. With respect to the threeinputgating devices numbered 101 and 111 in FIGS.

.24, these may be devices of the type manufactured and sold by Motorola,Inc. and designated by their Model No. MC792P. Finally, with respect toFIGS.

2-4, the four-input logic devices indicated by the numerals 113, 114 and1l5'may be of the type designated Model No. MC725P likewise currentlymanufactured and sold by Motorola, Inc.

The following are equations using conventionalsymbols and representingthe logic used in the currentcontrol system of FIGS. 2-4:

A Relay Combinations C Current Limit Settings F Frequency SwitchSettings Y and Z are used to simplify the above equations as fol- Themanner in which the various gating devices operate to convert the inputsignals representative of the frequency and current magnitude are shownstep by step as the signals are processed through the circuits of FIGS.2 and 3. In each case, the input or inputs and the output are identifiedfor each gating element or they are readily recognizable from the inputleads associated with each such gating device.

The final outputs from current control system 40 are 5 shown in FIGS. 2and 3 and then indicated as they are used to exercise a control functionby the circuit of FIG. 4. The outputs indicated at the right side of theFIG. 4drawing as frequency control outputs are all utilized in drivecircuits which are identified'by the numerals 1220 through l22e. Thedrive circuits 122a-122e actuate the solenoids A, B, CD and E which havealready been shown in the schematic of FIG. 1. Relay sequencing of therelays A through E as it is used to control the different resistorconnections within the variable impedance means 24 is best illustratedby the following tabular representation and equations:

RELAY SEQUENCING RELAYS A E Current Limit X 5 ohms D Relay X ohm E RelayFrom Chart above;

X, energizes when combinations A A are possible.

It will be seen from the above table that one example of the magnitudesof resistors for use in the control system may be given as follows:resistors 26, 28, 30, 32, 34 1 ohm; resistor 36 5 ohms; and resistor 3825 ohms. These values are to be taken by way of example and notconsidered as a limitation with respect to this feature. I Again withfurther reference to the drawing of FIG. 4, it will be seen that foreach solenoid A-E there is provided the drive circuit l22a-122e, eachincluding a drive transistor 124 which has its principal electrodesconnected in series with a suitable positive voltage source and aresistor 126. The output from drive trans'istor 124 is taken from itsemitter and used to fire an associated solid state triac switch 128. Theoutput of thetriac switch 128 is used to directly actuate one of thesolenoids A-E. In addition to the firing of the four resistorcontrolling solenoids A-E, additional logical.

control functions, for example, to connect an ammeter Am in circuit.

Reference is now made to FIG. 1 and to clarify the operation of thevariable impedance network 24 as it is finally controlled by the outputfrom the current control system 40. The values of the various resistors26 through 38 have already been indicated hereinabove. The normally opencontacts, closed through the operation'of the several solenoids A-E, areidentified by corresponding letters attached to the contact for each.Thus it will be seen that as the frequency of operation of the powersupply is increased and as the current magnitude and thus the length ofon-time is increased, there will be a significant change in the seriesresistance value that is included in circuit with the gap.

Now.with reference to FIG. 4, it will be seen that a provision is madein the current control logic system for providing a control inputdepending on whether the power supply is being operated as a singlepower or whether a pair of power supplies are being operated at thesametime, that is, in parallel to a single machining gap-with one or moreelectrodes used at the same time to machine one or more workpieces. Forthis purpose, the gates 118 are connected as shown, whereby the currentcontrol circuit is permitted to inhibit its own inputs and used toaqcept input signals from a second power supply, thus commanding theresistor combination which is appropriate in relation to the otherinput. It will thus be seen that the present invention provides a noveland highly effective current control system which has the function ofpredetermining and connecting the appropriate series resistance inaccordance with the frequency at which the power supply is preset and infurther dependence on the current magnitude that has been programmed.

' What is claimed is: v y

I. In an electrical discharge machining apparatus operable to providemachining power pulses of independently variable current and frequencyacross a machining gap;

a current control system including a plurality of different magnituderesistors connectable in series with said gap;

a first switching means presettable to provide a predetermined pulsefrequency;

a second switching means presettable to provide a predetermined gapcurrent; and v means for combining signals representative ofboth theaforesaid settings and deriving a control output for connectingdifferent ones of said resistors in. series with the gap to provide amaximum gap current level corresponding to each frequency setting;

said means for combining signals comprising a plurality of gating meansconnectedin cascade and operable to provide said output to maintain thedesired current level.

2. The combination as set forth in claim 1. wherein said switching meanscomprise a pair of deck switches, each selectively positionable inaccordance with the se lected pulse frequency and pulse current,respectively.

3. The combination as set forth in claim 1 wherein a plurality ofindependently actuable relays are included for controlling theconnection of said resistors with the gap.

4. The combination as set forth in claim 1 wherein said gating devicesare of the integrated circuit type incorporating resistor-transistorlogic elements.

5.-An electrical discharge machining apparatus including means forproviding machining power pulses of independently variable currentmagnitude and frequency to a machining gap, a current control systemincluding:

a plurality of resistors selectively connectable in series with saidgap;

a pulsing means for generating said power pulses including a firstswitching means presettable to provide a predetermined pulse frequencyand a second switching means presettable to provide a predetertheaforesaid settings comprising, a plurality of gating devices operablyconnected in sequence; a plurality of relays, each corresponding to oneof said resistors and controlling its series connection with said gap;and

means connected to the output of said combining means for providing acontrol output to said relays for actuating them to provide a maximumcurrent level for each different frequency and current set-. ting. i

6. The combination as set forth in claim 5. wherein said pulsing meanscomprises an astable multivibrator, said multivibrator including a pairof alternately operated switches, each having connected to it a variableresistor-capacitor network, wherein said first switching means isoperable to select the capacitor magnitude and wherein said secondswitch is operable to select the resistor magnitude.

7. A combination as set forth in claim 5 wherein both said first andsecond switching means comprise a deck type manually positionableswitch.

8. The combination as set forth in claim 5 wherein each of said gatingdevices is of the integrated circuit type and of the resistor-transistorconfiguration.

9. An electrical discharge machining apparatus operable to providemachining power pulses of independently adjustable current andfrequency, the combination comprising:

an electric output switch having one of its principal electrodesoperatively connected to the machining p a current control systemincluding a plurality of resistors selectively connectable in serieswith said machining gap and said principal electrode;

pulsing means for operating said electronic switch;

a first switching means connected to said pulsing means and presettableto provide a predetermined pulse frequency for said pulsing means;

a second switching means connected to said pulsing means and presettableto provide a predetermined, pulse current for said pulsing means; and Wa means for combining signals representative of both the aforesaidsettings and providing a control output for connecting different ones ofsaid resistors to provide a maximum gap current level corresponding toeach frequency setting said signal combining means comprising aplurality of gating devices connected in cascade and operable to providesaid control output to maintain the desired level of current magnitude.

10. The combination as set forth in claim 9 wherein a plurality ofrelays are included for connecting said resistors in series with the gapand said electronic output switch principal electrode.

11. In an electrical discharge machining power supply operable toprovide machining power pulses of independently variable frequency andcurrent magnitude to the gap, a current control system comprising:

sponding to each of said first and secondmeans and positioned inaccordance therewith;

means for combining the aforesaid signals and prosaid variable impedancemeans comprises a plurality of resistors selectively connectable incircuit with said gap n n l 13. The combination as set forth in claim 12wherein a-plurality of relays, each corresponding to at least one ofsaid resistors, are connected between said gating devices and said gapfor connecting said resistors to said gap. I o

14. In an electrical discharge machining apparatus 7 operable to providemachining power pulses of variable frequency and on-time to the gap, acurrent control system comprising:

a first means presettable to provide a predetermined pulse frequency;

a second means presettable to provide predetermined pulse on-time;

a read-out means operatively connected to said first and second meansfor providing a signal representative of their respective settings;

means for combining the aforesaid signals to provide a control outputsignal comprising a plurality of gating devices connected in cascade;and

a variable impedance means connected in series with said gap andcontrolled by said control output signal for maintaining a maximumcurrent level corresponding to each preset combination of frequency andon-time setting.

1. In an electrical discharge machining apparatus operable to providemachining power pulses of independently variable current and frequencyacross a machining gap; a current control system including a pluralityof different magnitude resistors connectable in series with said gap; afirst switching means presettable to provide a predetermined pulsefrequency; a second switching means presettable to provide apredetermined gap current; and means for combining signalsrepresentative of both the aforesaid settings and deriving a controloutput for connecting Different ones of said resistors in series withthe gap to provide a maximum gap current level corresponding to eachfrequency setting; said means for combining signals comprising aplurality of gating means connected in cascade and operable to providesaid output to maintain the desired current level.
 2. The combination asset forth in claim 1 wherein said switching means comprise a pair ofdeck switches, each selectively positionable in accordance with theselected pulse frequency and pulse current, respectively.
 3. Thecombination as set forth in claim 1 wherein a plurality of independentlyactuable relays are included for controlling the connection of saidresistors with the gap.
 4. The combination as set forth in claim 1wherein said gating devices are of the integrated circuit typeincorporating resistor-transistor logic elements.
 5. An electricaldischarge machining apparatus including means for providing machiningpower pulses of independently variable current magnitude and frequencyto a machining gap, a current control system including: a plurality ofresistors selectively connectable in series with said gap; a pulsingmeans for generating said power pulses including a first switching meanspresettable to provide a predetermined pulse frequency and a secondswitching means presettable to provide a predetermined gap currentmagnitude; means for combining signals representative of both theaforesaid settings comprising, a plurality of gating devices operablyconnected in sequence; a plurality of relays, each corresponding to oneof said resistors and controlling its series connection with said gap;and means connected to the output of said combining means for providinga control output to said relays for actuating them to provide a maximumcurrent level for each different frequency and current setting.
 6. Thecombination as set forth in claim 5 wherein said pulsing means comprisesan astable multivibrator, said multivibrator including a pair ofalternately operated switches, each having connected to it a variableresistor-capacitor network, wherein said first switching means isoperable to select the capacitor magnitude and wherein said secondswitch is operable to select the resistor magnitude.
 7. A combination asset forth in claim 5 wherein both said first and second switching meanscomprise a deck type manually positionable switch.
 8. The combination asset forth in claim 5 wherein each of said gating devices is of theintegrated circuit type and of the resistor-transistor configuration. 9.An electrical discharge machining apparatus operable to providemachining power pulses of independently adjustable current andfrequency, the combination comprising: an electric output switch havingone of its principal electrodes operatively connected to the machininggap; a current control system including a plurality of resistorsselectively connectable in series with said machining gap and saidprincipal electrode; pulsing means for operating said electronic switch;a first switching means connected to said pulsing means and presettableto provide a predetermined pulse frequency for said pulsing means; asecond switching means connected to said pulsing means and presettableto provide a predetermined pulse current for said pulsing means; and ameans for combining signals representative of both the aforesaidsettings and providing a control output for connecting different ones ofsaid resistors to provide a maximum gap current level corresponding toeach frequency setting said signal combining means comprising aplurality of gating devices connected in cascade and operable to providesaid control output to maintain the desired level of current magnitude.10. The combination as set forth in claim 9 wherein a plurality ofrelays are included for connecting said resistors in series with the gapand said electronic output switch principal electrode.
 11. In anelectrical discharge maChining power supply operable to providemachining power pulses of independently variable frequency and currentmagnitude to the gap, a current control system comprising: a first meanspresettable to provide a predetermined pulse frequency; a second meanspresettable to provide a predetermined gap current magnitude; a read-outmeans operatively connected to said first and second means for providinga signal representative of their respective settings; said read-outmeans comprising a deck switch corresponding to each of said first andsecond means and positioned in accordance therewith; means for combiningthe aforesaid signals and providing a control output signal; and avariable impedance means connected in series with said gap andcontrolled by said control output signal for maintaining a maximumcurrent level corresponding to each combination of frequency and currentmagnitude setting said means for combining signals comprising aplurality of gating devices of the resistor-transistor logic typeconnected in cascade.
 12. The combination as set forth in claim 11wherein said variable impedance means comprises a plurality of resistorsselectively connectable in circuit with said gap.
 13. The combination asset forth in claim 12 wherein a plurality of relays, each correspondingto at least one of said resistors, are connected between said gatingdevices and said gap for connecting said resistors to said gap.
 14. Inan electrical discharge machining apparatus operable to providemachining power pulses of variable frequency and on-time to the gap, acurrent control system comprising: a first means presettable to providea predetermined pulse frequency; a second means presettable to providepredetermined pulse on-time; a read-out means operatively connected tosaid first and second means for providing a signal representative oftheir respective settings; means for combining the aforesaid signals toprovide a control output signal comprising a plurality of gating devicesconnected in cascade; and a variable impedance means connected in serieswith said gap and controlled by said control output signal formaintaining a maximum current level corresponding to each presetcombination of frequency and on-time setting.